Field of the Invention
The present invention generally relates to a variable magnetization machine controller. More particularly, the present invention relates to a controller for changing the magnetization level of a variable magnetization machine, such as an electric motor employed in an electric or hybrid electric vehicle.
Background Information
Electric vehicles and hybrid electric vehicles (HEV) include an electric motor that operates as a drive source for the vehicle. In a purely electric vehicle, the electric motor operates as the sole drive source. On the other hand, an HEV includes an electric motor and a conventional combustion engine that operate as the drive sources for the vehicle based on conditions as understood in the art.
Electric vehicles and HEVs can employ an electric motor having variable magnetization characteristics as understood in the art. For example, the magnetization level of the motor can be increased to increase the torque generated by the motor. Accordingly, when the driver attempts to accelerate the vehicle to, for example, pass another vehicle, the motor controller can change the magnetization level to increase the torque output of the motor and thus increase the vehicle speed.
Certain techniques exist for increasing the magnetization level of such an electric motor. In one technique (conventional technique 1), only the d-axis current of the motor is increased. This results in the electric motor outputting a pulsating torque that pulsates due to the pulsating d-axis current and also due to changes in the magnetization level. In another technique (conventional technique 2), such as that described in JP 2009-153296, a controller can set the magnetization level of the motor based on values retrieved from a look-up table. In doing so, the controller can regulate the d-axis current and also the q-axis current. However, as understood in the art, because the q-axis current is determined using open loop (feed forward) control, a 2-dimensional look-up table is necessary to maintain a constant torque output by the motor. As further understood in the art, creating such a look-up table is very complicated and time consuming. Also, since open loop control is used, the same look-up table may not necessarily be useful with different motors due to the different characteristics of the individual motors, even if those motors are manufactured in mass production.